1. Field of the Invention
The invention relates to the field of bonding. More specifically, the present invention relates to methods and systems for semiconductor bonding like flip-chip bonding and corresponding 3D integration, as well as to devices thus obtained.
2. Description of the Related Technology
Flip-chip bonding often uses an electroplated solder material to bond to a base metal using a bonding temperature higher than the melting point of the solder. During the bonding process, the liquid solder quickly reacts with the base metal to form the intermetallic compounds (IMC) joint. For temperature sensitive structures, such as advanced DRAM and 3D chip stacking integrating different devices in various levels, bonding has to be carried out at lower temperatures below the melting point.
However, below the melting temperature, the solid surface is rough and this does not allow a good contact. Cu and Sn then can only locally react with each other to form a weak IMC joint. Compared with the evaporated or sputtered Sn and Cu, this becomes even more critical for electroplated materials that are often very rough. The weak joint will result in bad electrical contacts and low mechanical strength, and hence will often cause problems for the advanced DRAM process and 3D chip stacking technology.
In order to ensure full contact between the solder and base metal, either high bonding pressure or flat bonding interface is needed to put the electroplated solder and base metal together. To realize a flat bonding interface for good contact often additional equipment such as a fly-cutter is needed. Fly-cutting, whereby a diamond blade scans over a wafer surface, can be used to make the solder or base metal uniform in height. The micro roughness can also be reduced. However, it is difficult to well control the plastic deformation of the soft materials during the cutting process with a diamond blade, which will cause some misalignments for flip-chip bonding. It can even cause electrical shortcut for the fine pitch bump connection if the plastic deformation is too large. Also the cost of fly-cutting is relatively high.
Because of the high roughness of the solder and/or base metal, flip-chip bonding may require high pressure to realize good contacts. This has some important disadvantages. First, high pressure may damage devices, for example when (ultra) low-k materials with about 40% porosity are incorporated. Second, when a thinned device wafer that has some micro cracks generated during the wafer thinning process is used for 3D integration, high bonding pressure can enlarge the micro cracks and hence also damage the device. Finally, the mechanical thinning process can also generate some defects such as dislocations and voids. These defects will re-distribute under high bonding pressure and eventually influence the local stress condition and lower the device performance.
US2002/0027294 describes the use of the randomly distributed hard particles to realize electrical connections between two metal surfaces. In this case, hard particles are affixed to metal contact surfaces and a portion of the hard particles penetrates into the base metals with pressure. Therefore, the electrical connection is only realized locally through the particles. This limits the contact area, the electrical current that can flow and the reliability. Moreover, a non-conductive adhesive layer is used between the two base metals to provide the principal force for mechanical connection.
In U.S. Pat. No. 7,132,721 a method for bonding components of MEMS is described using a solid-liquid interdiffusion process. This method is used for pressure sealing. However, in case the boning temperature is lower than the melting temperature of the solder materials, an effective bond can only be realized after several hours. In addition, very homogenous metal layers are required in order to produce the hermetically sealing. Therefore, vacuum deposition technique is used to deposit the different metal layers.
U.S. Pat. No. 6,994,920 presents a method for fusion welding of two members, but in this case the interfaces of the two members are heated to a fusion welding temperature, so high temperatures are used such as to melt the interfaces for welding.
Jackson et al. (in IEEE Electronic Components and Technology Conference p 1472-1479 (2003)) mention that the reliability and the performance of a solder joint is highly dependent on the microstructure. The intermetallic layer is brittle and the thickness can increase upon ageing. Cracks formed in a solder joint can grow until both mechanical and electrical failure occurs. Furthermore, large CuSn needles extending into the bulk of the joint are mentioned as a problem. The intermetallic in this case is mainly formed at temperatures above the melting temperature of the solder material.